I've been doing 22 miles each way 3 times/week for the past 3 yrs. The five years before that, I did 18 miles each way 5 days/week. Both are mostly two lane highway. Once you get used to it, it's really no big deal.

Riding conditions rather than length would affect my decision the most -- open highway with some light rollers in the mild OR climate is much easier than urban cycling, something with wicked steep hills, or insanely cold temps or consistently strong winds. With my current riding conditions, I would definitely ride less often if the commute was over 30 miles each way.